Pelletizing dies are utilized by polymer manufacturers for the purpose of converting their product into pellet form for bulk shipment to fabricators of plastic components. These fabricators require that the pellets meet minimum shape and size specifications. The ideal pellet will have a good length to diameter aspect ratio with a smooth skin and no tails or strings of polymer stretching from the pellet. The ideal pellet will maintain this characteristic shape during all operations.
As the demand for polymers increases, larger production lines are desired by the manufactures to economize production of the pellets. As the pelletizing dies become larger, the percentage of pellets produced that meet the required quality standard is reduced.
Compounding the problem is the fact that fabricators of plastic components are tightening the specifications as to the definition of a good pellet. The pellets that do not meet fabricators specifications must be re-melted or sold off at greatly reduced prices.
Pellets of poor quality result for a variety of reasons, poor heat distribution, poor heat control, improper cooling effects, incorrect geometries in the polymer channels and orifices, and improper cutting conditions to name a few
Several companies worldwide manufacture pelletizing dies. They have achieved some improvement in quality with empirical or trial and error methods. However, such manufacturers have been unable to resolve the pellet quality problem to the satisfaction of the polymer fabricators.
U.S. Pat. No. 5,679,380 includes a discussion of issues of pellet quality. It was noted that the pellet quality is positively affected by keeping the thermoplastics to be pelletized above solidification temperature until their exit from the holes or nozzles of the pelletizing die plate. It is mentioned that this gives rise to problems due to the fact that the molten plastics must be cooled directly after the exit from the holes of the pelletizing die plate. The use of a cooling bath results in a considerable temperature drop from the outside to the inside of the pelletizing die plate. The prior art solutions focus on heat supply and transfer issues at the die as well as the wearing protection issues as to the die outer surface exposed to cooling fluid flow. The knives, which are also believed to be a factor leading to pellet quality problems are not discussed in detail.